1. Field of the Invention
The present invention relates to a liquid crystal display, and more specifically to an alignment layer for initial alignment of liquid crystal in a liquid crystal display.
2. Discussion of the Related Art
Ultra-thin flat panel displays include a display screen with a thickness of not more than a few centimeters. Of these, liquid crystal displays are currently used in a wide range of applications, such as notebook computers, monitors, spacecraft, and aircraft. Liquid crystal displays have advantages of low power consumption due to low operating voltage and ease of portability.
A general liquid crystal display includes a color filter substrate having a color filter layer formed thereon, a thin film transistor substrate arranged opposite to the color filter substrate and having thin film transistors formed thereon, and a liquid crystal layer interposed between the two substrates.
The alignment direction of the liquid crystal layer in the liquid crystal display is varied depending on an applied voltage, which controls the light transmittance, thereby displaying images on a screen. To apply a voltage, electrodes are formed on the thin film transistor substrate and the color filter substrate. Specifically, pixel electrodes are arranged on the thin film transistor substrate, and a common electrode is arranged on the color filter substrate. A vertical electric field may be applied between the two substrates (in the case of a twisted nematic (TN) mode liquid crystal display), or a horizontal electric field may be applied between the pixel electrodes and the common electrode arranged on and parallel to the thin film transistor substrate (in the case of an in-plane switching (IPS) mode liquid crystal display).
FIG. 1 is an exploded perspective view of a general TN mode liquid crystal display according to the related art. As shown in FIG. 1, gate lines 12 and data lines 14 crossing each other are formed on a thin film transistor substrate 10 to form intersections. Thin film transistors T are formed at the intersections, and pixel electrodes 16 are connected to the thin film transistors T. In addition, a light-blocking film 22 is formed on a color filter substrate 20 to prevent light from leaking, an RGB color filter layer 24 is formed on the light-blocking layer 22, and a common electrode 25 is formed thereon. Thereafter, the substrates 10 and 20 are attached to each other to form one liquid crystal panel in which a liquid crystal layer is formed between the substrates 10 and 20.
When a vertical electric field is generated between the pixel electrodes 16 formed on the thin film transistor substrate 10 and the common electrode 25 formed on the color filter substrate 20, the alignment direction of a liquid crystal is controlled. If the liquid crystal layer is randomly arranged between the substrates 10 and 20, liquid crystal molecules included in the liquid crystal layer are not arranged in a fixed direction. Hence, although not shown in FIG. 1, an alignment layer for initial alignment of the liquid crystal is formed between the thin film transistor substrate 10 and the color filter substrate 20.
Formation of such an alignment layer for initial alignment of a liquid crystal has predominantly been achieved by rubbing. This rubbing method includes the steps of applying an organic polymer, such as polyimide, to a substrate to form a thin film, curing the thin organic polymer film, and rolling a rubbing roll covered with a rubbing cloth to rub the thin organic polymer film and to form an array of chains of the organic polymer in a particular direction.
With the use of the alignment layer formed with this rubbing method, liquid crystal molecules are aligned in the direction that the array of chains of the organic polymer are formed. That is, the movement direction of the rubbing roll is the same as the alignment direction of the liquid crystal molecules.
However, rubbing alignment method has the following disadvantages. First, when the arrangement of a rubbing cloth is non-uniform, light leakage may occur. FIG. 2 is a perspective view showing a state wherein the arrangement of a rubbing cloth is non-uniform.
As described above, structures, such as thin film transistors, color filter layers and electrode layers, are formed on a substrate. As shown in FIG. 2, when a rubbing roll 30 is rolled on the structures formed on a substrate 10 or 20, parts 32a of a rubbing cloth 32 covering the rubbing roll 30 may be non-uniformly arranged. This non-uniform arrangement of the rubbing cloth 32 causes a non-uniform array of chains of an organic polymer in regions of the substrate rubbed by the parts 32a of the rubbing cloth. As a result, the alignment of the liquid crystal is not uniform, thus causing light leakage.
Second, when a rubbing cloth does not come into contact with a substrate, light leakage may occur. FIG. 3 is a perspective view showing the arrangement state of a liquid crystal in the case where a rubbing cloth does not come into contact with a substrate.
As discussed above, electrode layers, such as pixel electrodes and a common electrode, are formed on a substrate. Due to a step height in electrode layers formed on a substrate 10, as shown in FIG. 3, a region (region “A”) is formed where a rubbing cloth 32 does not come into contact with the substrate 10. In this case, the alignment of a liquid crystal is not uniform in the region A, resulting in light leakage.
In conclusion, according to a related art rubbing alignment method, when the arrangement of a rubbing cloth is non-uniform or a rubbing cloth does not come into contact with a substrate, rubbing cannot be performed well, causing the problem of light leakage. The above-mentioned problems of the related art rubbing alignment method are attributed to a physical contact between a rubbing roll and a substrate. Thus, there exists a need for a novel liquid crystal alignment layer forming method to solve the problems of the related art rubbing alignment method.